beanbag heat sink - more comfortable than a bench?

I am new to the forum and have been researching the idea of a thermal mass heater for a month or two. The cob benches look nice and have interesting sculpture possibilities, but the idea popped into my mind today to store at least some of the heat in something much softer: a bag of pebbles. In other words, a bean-bag chair.

Has anyone tried this? Would the ducting be impractical?

My family still need convincing, but boy do I like brainstorming!

I would use at least a few inches of solid cob cause sinking it to far and sittin my bony end right on the pipe would scare me, but something like a bag of oat or rice hulls for a pad should pass the warmth and add to the thermal mass

It's important to understand the difference between insulation and thermal mass.

For a thermal mass heater, the bench has to absorb, hold, and radiate heat. For that you need solid material that conducts heat well.

Spaces within the material, such as you find in foam, wool batting, or indeed in a beanbag, don't conduct heat as well—they insulate. I think even a bag of pebbles will be less effective as thermal mass. The pebbles closest to the heat duct would absorb the heat and then radiate it into the air spaces around them slowly over time. Eventually some of the heat would get picked up by the other pebbles, but much of it would be lost to entropy by the time it reached the outermost pebbles. Most of the heat would never reach your butt, in other words.

A similar problem occurs with padding the bench with rice or oat hulls; while they have some thermal mass, they're also insulation. But a thin cushion of them is not much insulation, so you would still feel a good amount of the heat once the cushion eventually warmed up. And if you knew you were going to want a cushion, you could compensate by locating the pipe closer to the surface of the cob bench.

Wazabooz wrote:
I am new to the forum and have been researching the idea of a thermal mass heater for a month or two. The cob benches look nice and have interesting sculpture possibilities, but the idea popped into my mind today to store at least some of the heat in something much softer: a bag of pebbles. In other words, a bean-bag chair.

Perhaps gel would transmit heat better. I am not sure what kind of gel would not have problems with heat cycling though.

Thanks for the pointers! Maybe I shouldn't have said "beanbag" because I realize they're often made with styrofoam pebbles or something. The rock pebbles would probably not be as comfortable as I first thought.

So cushions are typically used with these benches? Works for me. The main issue here (apart from family resistance) is a very small living space. I can't waste half the main room on something that's not used in summer. A cob heat exchanger would have to double as shelves or something. Or, god forbid, I'd have to get rid of my piano.

The great thing about thermal mass is that it's nice and cool in summer, so if you tend to have cooling bills at that time of year, it could reduce them, or at least make the room more comfortable in the hotter part of the day. A cob bench could be a breakfast nook or just every day seating. If you have a couch in the room, imagine the space available if the couch were built into the wall instead. If you're not using the mass heater for heat, you can layer it with all the cushions you want.

For comfort and some amount of thermal mass, consider a pad filled with sand. You might want to test the thermal properties of coarse vs. fine grained sand. I've also heard of folks who build thermal mass heaters where the mass forms the base of their bed, so some substantial padding should work if the design takes it into account.

But if it doesn't work for your application, don't force it! There are lots of other options. RMH also aren't ideal for settings where you wouldn't be firing it every day or at least every two days, regularly, for a few months out of the year... they don't heat up quickly, so getting them up to temperature after they've been allowed to cool down, for instance if the weather repeatedly warms up and then gets cold again over the course of your winters, negates their efficiency to some degree.

Wazabooz wrote:
Or, god forbid, I'd have to get rid of my piano.

Maybe try doing without a leg first, keep the piano. Get rid of a TV or something.

The "sand" they sell for ashtrays is silicon carbide, which has extremely high thermal conductivity.

It will chew through fabric faster than ordinary sand would, if used as a cushion,  because it's so sharp and hard, but it still might be worth considering.

For the summer, I could imagine hooking up the exhaust of your RMH to a solar chimney, and the intake to the basement or some other source of cooler air. On clear nights, that same solar chimney will radiate heat out very quickly, and air will flow backward through the system, chilling it a little extra before dawn.

You could try a layer of polyachrilymides (water holding gel) sewn into a cushion.  These are used currently to heat or cool the body sewn into vests and neck/headbands.